The present invention relates to photosensitive imaging systems, and particularly diazonium vesicular imaging systems.
Vesicular images are formed in a photosensitive film by small bubbles or vesicles of gas which are formed and trapped in the areas of the film exposed to light. Generally speaking, the film has a colloid or a resin coating, referred to as a vehicle, on a backing material and a light-sensitive agent, most commonly a diazo compound, dispersed throughout the coating. When the film is selectively exposed to imagewise distributed radiation, the light-sensitive agent is decomposed and releases molecules of a gas (nitrogen in the case of diazo compounds). The gas ordinarily may be dissolved in the coating and does not form vesicles immediately, but does so when the film is developed by heating, presumably because the vehicle is softened sufficiently on heating for the gas molecules to form bubbles in the vehicle and for the bubble to expand. The resulting vesicles make the vehicle opaque to transmission of light in the imagewise exposed areas. The vesicles also reflect and scatter light so that they appear white.
Early vesicular materials employed gelatin as the vehicle. These suffered from the difficulties of slow speed, short-lived latent images, and rapid fading of the vesicular images. Later work revealed that this last problem was caused, in part, by the sensitivity of gelatin to water. Gelatin vehicles absorbed moisture from the atmosphere and became soft, thus collapsing the vesicles and destroying the image.
It is now preferred to employ polymers or resins as the vehicle. Vehicles which are particularly preferred include those described in U.S. Pat. Nos. 3,620,743 and 3,622,336. U.S. Pat. No. 3,620,743 discloses a vehicle made from a water-insoluble polymer selected from a group consisting of homopolymers of .alpha.-chloroacrylonitrile and copolymers of .alpha.-chloroacrylonitrile with a different vinyl monomer in which the mole fraction of the vinyl monomer in the copolymer is less than 0.50. U.S. Pat. No. 3,622,336 discloses a vehicle which is a copolymer of .alpha.-chloroacrylonitrile and .alpha.-methacrylonitrile.
It would be desirable to use water-soluble vehicles in the manufacture of vesicular films as this would remove the cost of organic solvents and reduce the manufacturing costs and health risks involved in working with organic solvents. Previous attempts to use water soluble resins such as poly(vinyl alcohol) have sought to prevent penetration of moisture into the vehicle by the application of a water-impermeable topcoat. This construction somewhat reduced the moisture sensitivity of the film, but images would still shortly fade because of the collapse of vesicles under ambient humidity conditions.